This Nonprovisional application claims priority under 35 U.S.C. §119(a) on Patent Application No(s). 092105340 filed in TAIWAN on Mar. 12, 2003, the entire contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a method of forming a dual-layer resist and application thereof and in particular to a method of forming a patterned resist layer on another patterned resist layer.
2. Description of the Related Art
As the size of memory cells of mask read only memory devices is reduced, defining a code implantation area in a memory cell using a single mask and single photolithography process is more difficult. Thus, a manufacturing process employing two masks and two photolithography processes has developed. FIG. 1A is a schematic top view showing formation of an NMOS in a memory cell and FIG. 1B shows a schematic cross section along line AA′ in FIG. 1A. Buried bit lines 10 and word lines 12 are staggered and the position of a word line between every two buried bit lines acts as an NMOS. When a coding-related process is performed, a hard mask layer 14 (such as silicon dioxide) is first deposited, then, referring to FIGS. 2A and 2B, a photolithography process is performed (using a pre-code mask) to roughly form parallel resist lines 16 on the buried bit lines. Next, part of the hard mask 14 not shielded by the resist lines 16 is removed, and the resist lines 16 are then removed. Referring to FIGS. 3A and 3B another resist layer 18 is then formed using another photolithography process (using a code mask) to define memory cells for code implantation. The logic state of the area not shielded by the resist layer 18 and the hard mask 14 will be determined by the subsequent ion implantation process.
As is known from the preceding description, after an NMOS of a memory cell is formed and before the coding is accomplished, there are at least two photolithography processes, one deposition process, one etching process, one resist removal process, and one ion implantation process. Such a manufacturing process is costly and complicated but necessary, because if a subsequent resist layer is directly coated on the previous resist layer, the defined pattern on the previous resist layer will change due to the dissolution of the previous resist layer in the solvent used in the subsequent resist layer. Therefore, a hard mask layer is required to transfer patterns.